57353235-Pig-Trap-Design-Consideration.pdf

Considerations In Pig Trap Design By Duncan Warriner, Pipeline Engineering, North Yorkshire, Yorkshire, UK

ig trap design choices can be vitally important in ensuring that the operational unit is practical and safe to use. It is useful to review what is expected from a pig trap and the role of the various key trap components. Pig traps provide a means of loading and unloading pipeline tools, including intelligent pigs, into pipelines. They take the form of a vessel that can utilize diverted flow for either launching or receiving. The vessel can be isolated from the pipeline to facilitate the loading and unloading of pipeline tools. Because the trap when in use

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is essentially part of the pipeline, it is important that it be capable of withstanding the pipeline conditions such as pressure, temperature and the effects of the service medium i.e., corrosion. The trap or vessel incorporates a number of nozzles at suitable locations for the diversion of flow, draining, venting and monitoring. The diversion of flow provides the motive force to move the tool through the vessel neck and into the pipeline flow. The vessel neck is critical in that it must be of a compatible internal diameter to the pipeline itself. Once it has been isolated, the vessel can be opened in order to either load or unload a pipeline tool. The opening is at the other end of the vessel from the neck to the pipeline connection and is generally of a larger diameter than the neck to provide ease of loading for launching and an increase in bypass for receiving. This

Pip el in e & Ga s Jou rn al / Augu st 200 8 / www. pgj onl ine .co m

Rapid opening closure.

opening can take the form of a simple flange blind flange arrangement or a more complex quick opening hub-door arrangement more commonly known as a quick opening closure. For subsea applications, the same basic principles apply apply.. Howev However, er, the trap may require additional protection for the valves and pipework pipework in the form of a shielding frame

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Pi pe li ne & Ga s Jou rn al / August August 2008 / www.pgjonline.com

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to reduce the risk of damage from f ishing nets and ships’ anchors. It is beneficial for subsea traps to be either temporary or removable to facilitate loading and maintenance. In addition, for unmanned or particularly hazardous or frequent pigging operation applications it is often advantageous to consider multiple launching capabilities. This can be via multiple valve/mechanical methods or by more recent AMPL-type technology.

Mechanical Design Considerations In the first instance we need to understand the applicable design code. As pig traps are pressure vessels, pressure vessel design parameters must apply. However, unlike pressure vessels, pig traps must be capable of retaining pressure while allowing tools to be launched and received. It is these tools which determine the overall trap dimensions. Certain tools such as the metal-bodied intelligent pigs are both heavy and long and must be considered, along with the internal pressure requirements, when designing supports, lifting lugs and assessing foundation loadings. Trap design must also satisfy applicable codes, whether it be PD5500, EN13445, ASME VIII (Div 1 or 2), ASME B31.4,

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B31.3, B31.8, AS2885 et al. While essentially attached directly to the end of a pipeline as an isolatable extension, the trap can often be designed to a vessel code rather than a pipeline code. This in itself can often provide a significant challenge in meeting the pipeline internal diameter with a t hicker calculated vessel wall. In this case, we have what is called a specification “break” between the pipeline and trap; in other cases the trap can be designed to the same code as the connecting pipeline. Whichever approach, the designer must ensure all appropriate loadings and conditions are addressed to produce a safe working design. Typical basic design parameters to consider include design code, design pressure, design temperature, materials, external loadings and cyclic requirements. It is useful to r un through each of these parameters. Design Code. This can be any of the national or international standards and either a specific pressure vessel code or pipeline code. However, we should ask ourselves what a code is for. A code is a statement of the minimum requirements needed to protect the community. “Community” is generally interpreted to include the users of the equipment, the general public and the wider environment. environment. A code is not a complete guide to design. A design can meet the requirements of a code and


still be a bad design for several reasons. It may be costly, dangerous, wasteful, etc. Codes have an important role in securing minimum standards for protection of the community, but should not be forced forced into role roless they they were were not not intende intended. d. Pressure. This can be client-specific or based upon the ASME/ANSI Pressure/ Temperature Class tables. The design pressure should never be less than that of the pipeline. Temperature. The design would take account of the maximum design temperature, but also the minimum design temperature in terms of material selection. Materials. Often specified by the client, however, where possible it is better to allow the trap manufacturer to select materials that are compatible with the pipeline materials and meet the design specification requirements. This approach ensures that the most suitable, cost-effective and readily available materials that fully meet the design requirements are selected. It is important that the line product is specified and whether it is sour, toxic or corrosive. This will influence the selection of not only the metallic elements, but also the elastomeric materials, which typically constitute the closure-sealing element. All components in sour service should be specified to meet the requirements of NACE MR 01 75 for sour service and where the product is very sour, con-

sideration should be given to specifying that the materials should be HIC/SSCC-tested. Finally, any material must be compatible with its mating material in respect of weld ability, wall thickness, chemical and mechanical properties. External loadings. These loads consist of those imposed by attaching pipework acting upon nozzles. In addition, external pressure due to water depth in a subsea application could fall into this category category.. Cyclical requirements. Should the unit be subject to frequent pigging operations, this may in turn create sufficient cyclical loading to warrant fatigue analysis. In addition, we need to consider support loads, wind, blast and seismic loadings, loads induced during transportation of the units and any subsequent lifting lug requirements.

Process Design Considerations As specialist pigging designers we look at the requirements from several angles. In addition to an examination of pipeline configurations, pressures and diameters, the product and its temperature and flow rates, we will seek to define precisely what the systems have to do. Will the tool(s) travel in one direction only, forward or backward, or is two-way motion needed? Other questions include: Is propul-

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sion liquid, gas or both? Will the tool(s) be used for cleaning, stabilization, separation or a combination thereof? How frequently will the tool have to travel and how far? What is the performance of the line-pumping systems? Often it is necessary to consider the suitability of industry norms to specific applications, particularly with regard to such parameters as nozzle sizing. Industry norms for nozzle sizing do not always consider the practical issues associated with the service medium. For instance, the number and size of drain nozzles will differ for a dry gas medium as compared to a waxy crude medium. In the case of waxy crude it may be necessary to consider larger drain nozzles located at the closure and reducer end of the major barrel. The pigging philosophy itself may also have an impact on the nozzle size. For instance, when receiving, receiving, if the pig is not stopped at the bypass barred tee and brought into the vessel at a lower velocity than that of the medium (with partial bypass via the line barred tee), the bypass line may not be of sufficient diameter to carry the flow. In this case, the bypass size may need to be increased. Many questions have to be considered before a design solution begins to emerge. A thorough analysis is essential to design traps capable of absorbing all the necessary forces and loadings.


Practical Design Considerations In addition to mechanical and process considerations, the trap designer must consider practical issues such as location, layo layout, ut, environment and logistics.

Trap Components The basic trap components are barrel, reducer, neck pipe, branch nozzles, kicker, drain, vent, blowdown, balance line, pressurizing line, thermal relief valve, pressure indicator (pressure gauge), supports, lifting lugs, earthing (grounding) lugs, end closure and pig signaller. Barrel. This is the major section of the trap and the means by which the loading and unloading of pigs can be carried out with com parative parati ve ease and safety. safety. It is usually equipped equipped with an opening closure at one end while its other end is welded to a reducer. For conventional pigging its diameter is usually a nominal 50 mm above the line size, while for intelligenttype pigs it is recommended that it be at least a nominal 100 mm above the line-pipe diameter. The barrel length is dependent upon the operating procedures, service, type of pigs, available space, etc., but in general for launchers deploying conventional pigs - 2 x pig length, while for receivers - 3/3.5 x pig length.

When deploying intelligent-type pigs, the barrel length should be decided only after consultation with the pig manufacturer. manufacturer. Reducer. This is welded at its larger diameter to the other end of the barrel, and can be eccentric or concentric. Generally, horizontal units use eccentric on launchers, concentric on receivers. Vertical units use concentric throughout. Neck Pipe. This is the minor diameter section of the pig trap and is welded at one end to the smaller diameter of the reducer and terminates in either a butt weld end preparation or flange at its other end. Dimensionally it is usually very short - one pig length + 10%, however, in the case of intelligent pig receivers it could be as long as four meters. Branch Nozzles. On the simplest of traps there can be as few as four nozzles - kicker, vent, drain and pressure indicator. On more complex traps, additional nozzles are incorporated for, blowdow blo wdown, n, balance balance,, equaliz equalizing, ing, pressur pressurizing izing and thermal relief valves. Other nozzles may be fitted in accordance with the clients’ specific requirements, but consideration should be given to the design requirements regarding proximity of welds etc. Running through the various nozzles, the size and positions are generally in accordance with the following. Kicker. This nozzle is situated on the barrel

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at the closure end in the case of launchers, and near the reducer end in the case of receivers. In the case of universal (bi-directional) traps, a single connection midway along the barrel or twin connections, one in the launch position and one in the receive position, can be fitted. Kicker connections should not be positioned at the 6 o’clock position; historically, this position causes damage to the pigs. Drain. This nozzle should be situated near the closure end for horizontal traps and near the neck flange on vertical launchers. For receivers, receiver s, a drain point near the barrel reducer is recommended, or alternatively on the neck pipe near the end flange. For receive receivers rs which are sloped for spheres, two drain points should ideally be specified and located together near the closure end, but separated by half a sphere diameter, this prevents the drains being blocked by the spheres. Size of drain connections should be not less than two inches for traps up to 14 inches line size and four inches for traps above that. Vent. This nozzle should be situated near the closure end or highest point; a fur ther connection may be considered near the trap neck end flange to ensure depressurization behind the pig in case it becomes stuck in the neck pipe. Size of vent connections should not be less than 1/2 inches ns.


Blowdown. On high-pressure gas systems consideration should be given to the provision of a blowdown line, incorporating a globe valve or restriction orifice, for controlled depressurization. Size should not exceed two inches ns. Balance line. This can be provided on launchers to enable filling and – pressurization of the barrel on both sides of the pig at the same time. This is to prevent a pig which is ready to be launched from moving forward and thereby hitting and possibly causing damage to the trap valve, or backwards and losing the seal in the reducer. Consideration should also be given to the provision of a balance line on the receiver to prevent any possible pressure differential across a receiving pig. Size should be in the region of two inches ns. Pressurizing line. This may be required around kicker valves for several reasons speed of operation, control of barrel - pressurization and/or damage to the kicker valve seats or other internals. Similarly Similarly,, a pressurizing line around bypass valves should also be considered, for equalizing possible high-differential pressures. Size should be smaller than the balancing line connection. connection. Thermal relief valve. This can be provided at locations where it is anticipated shut-in or trapped fluid could exceed the design pres-

sure. Size would be as dictated by design conditions. Pressure indicator. This should be fitted toward the closure end and visible to the operator, may be incorporated with the vent connection. Size should be in the region of 1/2 inch to one inch, threaded threaded,, with four inches or six inches dial size. Supports. Required to permanently support and restrain the pig trap, supports should be designed to carry the weight of the pig trap system filled with water (or other fluid if their density is greater), together with the weight of the associated heaviest pig. Supports under the barrel should normally be of the sliding type to compensate for expansion of the unrestrained part of the pipeline. Other supports may be fixed if design calculations indicate that sufficient flexibility is incorporated in the pipework to compensate for any axial and transverse movements. Lifting lugs. These are required to facilitate lifting of the complete trap during the installation stage. Grounding lugs. These are required to reduce the build-up of static electricity. Static is a seriously under-estimated yet ever-present hazard. Being invisible to the naked eye, it tends to be ignored. Yet, an undischarged build-up of electrostatic can take hours, even days, to relax back into equilibr equilibrium, ium, resulting resulting in a potentially potentially

lethal workplace. If the accumulated static is suddenly discharged within a hazardous atmosphere, the resulting spark may easily act as the ignition source for an explosion. End closure. This can be as simple as a flange and blind, but is more commonly found in the form of a quick opening door, an assem bly which prov provides ides a quick, easy and safe access to the barrel when open and seals the bore when closed. The closure should should be fitted

at a minimum with a safety bleeder device, forming part of the door-locking mechanism, to safeguard personnel before and during door opening. An interlocking system between the various valves and the closure may be considered as an additional feature. Pig signaller. This is also known as a pig detector, a device set on or into the pipeline which indicates the momentary presence of a pig at a prec precise ise loca location tion.. Sign Signalle allers rs sho should uld be

Rapid End Closure

Pig signaller-set, left, and pig signallertripped, right.

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installed on both sides of the trap valve. For launchers, it should be positioned on the main pipeline pipe line at a dist distanc ancee from the trap valve of at least the length of the longest pig anticipated. For receivers, it should be positioned on the trap neck pipe at a distance from the trap valve, again, equal to the anticipated longest pig length.

Generic Types A pig trap can take a number of forms but generally falls into the generic types such as bidirectional, vertical launcher, vertical receiver, horizontal launcher, and horizontal receiver receiver..

Innovations Clearly at the forefront of any design considerations is the issue of health, safety and environment. Any reduction in manual intervention or potential environmental risks is advantageous and we should look to achieve this where possible possib le for very freque frequent nt pigging pigging opera operations tions or those in very remote/hazardous locations. This reduction in the number of times the vessel needs to be opened can be achieved by the use of multiple launching/receiving capabilities. There are four main systems available: Valve type Pig Launcher. Vertical Multiple Pig Launcher. Automatic Sphere Launcher. Automatic Multiple Pig Launching (AMPL).

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Pig launcher

Valve Type Multiple Pig Launcher. The trap is fitted with a set of launch valves for each pig in the launcher. This allows line pressure to be directed behind each pig in turn and


launched individually as required. Although a very reliable system, the additional valve requirement adds considerable considerable cost to the system, especially when fitted to a large-diamete large-diameterr

pipeline, due to the high cost of the large valves required for the multiple kicker lines. This system is used predominantly in subsea applications with either a diver or ROV operating the valves as required, or the valves are fitted with remotely operated actuators which adds to the overall cost. It is also retrofitted to existing installation pig launchers if an automatic launching requirement is later identified.

Multiple Pig Launcher

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Vertical Multiple Pig Launcher. This system is based around the space-saving solution of a vertical pig launcher used on many offshore installations. It is basically an extended standard launcher, but with the addition of hydraulically operated launch pin s tha t pro tr ude int o the overs ize bar rel of the launcher. The first pig is loaded into the throat of the launcher, and then the first or lowest launch pin is extended. Another pig is the n loa ded — tha t is — hel d up by the launch pi n. S ub se qu en t pins pi ns an and d pi gs ar aree then inserted until the trap’s capacity is reached. The first pig is launched in the normal way. When another pigging run is required, the lowest launch pin is retracted, and the pig abovee it f alls into the abov throat of the launch-


er. This pig is then launched. Subsequent pins are then retra retracted cted as and when furt further her pigging piggin g runs r uns are requir required. ed. As mentioned earlier, this is a very reliable multiple pig launching system. However, the initial cost outlay can be quite high due to the launch pins and the hydraulics required to operate them. Extra maintenance is needed to ensure the launch pins oper operate ate cor rect ly. They als also o requ require ire regular inspections to ensure they have not become beco me bent due to exces sive for forces. ces. Automatic Sphere Launcher. Although not strictly a pig launcher by definition, the principles are the same. Spheres are regularly used when large numbers of cleaning runs are required,, but the eff iciency is not critical, usurequired ally where the removal of unwanted fluids is the primary cleaning requirement. Automatic sphere launchers operate similarly to the Vertical Multiple Pig Launcher, utilizing launch pins, but with the trap on a slight decline from the horizontal. Spheres are loaded with the front pin extended and the rear pin retracted. When a launch is required, the rear pin extends, holding back the remaining spheres. The front pin retrac retracts, ts, allo allowing wing the lead sphere to roll

Sphere Launcher

Pipeline Engineering to meet the need for a multiple pig-launching system that can be retrofitted to existing launcher facilities without the need for costly trap modifications. The system operates in a similar way to standard pigging equipment. The pigs are either preloaded into a specially designed cassette, which is then inserted into the standard trap, or they are loaded into the trap in which the cassette has already been fitted. The number of pigs in the cassette is pre-determined by the length of the trap.

Inspection into the pipeline. When the downstream signaller indicates the sphere has been successfully launched, the front pin extends, the rear pin retracts, the remaining spheres roll forward and the system resets itself for the next launch. Although widely used, this system is predominantly used for fluid removal and not for physical line cleaning due to the sphere only having one sealing face, and therefore a tendency to ride over solid residue in the pipe. Automatic Multiple Pig Launching System (AMPL). This system has been developed by

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Having made all the necessary considerations during the design phase and ensured


compliance by manufacturing inspection, it is important to inspect and maintain the units during operation. ACKNOWLEDGMENT:

This article is based on a presentation at the 20th international Pipeline Pigging & Integrity Management Conference sponsored by Clarion Technical Technical Conferences, Feb. 13-14, 2008, in Houston, TX. Author: Duncan Warriner

is general manager of metal products at Pipeline Engineering in the UK. He is a Chartered Mechanical Engineer with 19 years of experience dealing with the mechanical requirements and materials used in the oil and gas, construction and service industries, with specialist knowledge in pipework and pressure vessel design, fabrication and material selection.

Automatic Multiple Pig Launcher

57353235-Pig-Trap-Design-Consideration.pdf

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